1. Field of the Invention
The present invention relates to a data conversion method for converting digital data to signals suitable for the recording system or the transmission channel used when recording or reading the digital data onto or from a magnetic tape, and a recording/reproducing apparatus employing the data conversion method. The present invention further relates to encoding multiple kinds of data and data blocks of different lengths on the tape with a high density.
2. Description of Related Art
Prior art data conversion methods employed in magnetic recording/reproducing apparatus include, for example, an 8/10 modulation method such as disclosed in "THE DAT CONFERENCE STANDARD" (issued June 1987). The 8/10 modulation method is a data conversion method in which digital data is partitioned into datawords of 8 bits each for conversion into 10-bit codewords. FIG. 1 is a circuit block diagram for explaining this data conversion method, and FIG. 2 is a data conversion table used for the same. In FIG. 1, the reference numeral 1 designates an encoder for accepting eight-bit digital data and a one-bit table select signal (Q') at its respective inputs and for outputting a total of 11 bits, i.e. a 10-bit codeword plus a one-bit signal (Q) for selecting the table for the next codeword. Further, the numeral 2 denotes a flip-flop for delaying the codeword table select signal by one dataword. The encoder 1 includes a read-only memory (ROM) or the like which contains the data conversion table shown in FIG. 2, wherein codewords of CDS (Codeword Digital Sum)=0 are mapped on a one-to-one basis to 256 datawords from "OO" to "FF" of hexadecimal numeral while in the case of codewords of CDS=0, pairs of code-words, one with CDS=+2 and the other with CDS=-2, are each mapped to one dataword, the table of Q'=-1 consisting of codewords of CDS=+2 and the table of Q'=+1 consisting of codewords of CDS=-2. The table select signal (Q) is used to select the CDS (the table) having the direction that suppresses the dispersion of charges in the codeword sequence.
The operation of the above circuit will now be explained with reference to the timing diagram of FIG. 3. In FIG. 3, the reference signs (a), (Q), and (b) correspond to inputs/outputs at the respective parts shown in FIG. 1, and the reference signs (c) and (d) respectively represent an output signal from an NRZI converter (not shown) and a DSV (Digital Sum Variation) value at the end of each codeword.
First, an eight-bit dataword "FF" is input to the encoder 1, along with the table select signal (Q')=-1, and consequently, the encoder 1 outputs a 10-bit codeword "1111101010" of CDS=+2 corresponding to "FF" for Q'=-1. At the same time, the table select signal Q=-1 is output for the next codeword. The parallel 10-bit signal is then converted to a serial signal, after which the signal is NRZI-modulated. As a result, the DSV value at the end of the codeword becomes +2.
Next, when "00" is input to the encoder Is the encoder 1 outputs Q=1 together with a 10-bit signal "0101010101" of CDS=0 corresponding to "00" for Q'=-1 which is produced by introducing a one-symbol delay in the previous output Q=-1. As a results the DSV value at the end of the codeword after NRZI modulation remains at +2.
Next, when "11" is input to the encoder 1, the encoder 1 outputs Q=-1 together with a 10-bit signal of CDS=-2 corresponding to "11" for Q'=1. As a result, the DSV value at the end of the codeword after NRZI modulation becomes zero. In this manner, for each eight-bit dataword input to the encoder 1, a codeword to be output is selected from the table of either Q'=-1 or Q'=1 corresponding to the dataword on the basis of the table select signal output previously. The DSV at the end of each codeword after NRZI modulation can only take the value 0, +2 or -2. This means that the DSV dispersion is suppressed, as a result of which DC-free data conversion is realized.
As described above, according to the prior art data conversion method, eight-bit data is converted to a 10-bit codeword of CDS=0 or CDS=+2 or -2, and a DC-free signal is produced with the DSV dispersion suppressed, thereby minimizing intersymbol interference on the transmission channel and thus increasing the recording density per track. However, for recent digital magnetic recording/reproducing apparatus using a rotary head, a recording density as high as several square micrometers per bit is demanded, which necessitates not only increasing the recording density per track but also reducing the track width down to several micrometers. To implement such apparatus, it is highly useful to employ a dynamic tracking following (DTF) control system whereby pilot signals for tracking are recorded on the main track recorded by the rotary head and the playback head is controlled to follow the recorded track curves during playback. When the prior art data conversion method is employed in such apparatus for multiplex recording of the pilot signals, the digital signal spectral distribution has to be obtained down to ultra low frequency ranges although the recorded information signals contain no DC components; the resulting problem is that the pilot signals cause external disturbances, leading to increased errors in the detection of the digital signals.
One possible approach to overcoming the problem of the pilot signals causing external disturbances to the digital signals may be generating pilot signals synchronized to the digital signals. However, the prior art data conversion method is effective only in suppressing the DSV dispersion and is not capable of actively controlling the DSV, and therefore, has the problem that it cannot generate pilot signals synchronized to the digital signals.
FIG. 4 shows a DAT recording format employed in a magnetic recording apparatus using the 8/10 modulation method. As shown, according to the format of FIG. 4, ATF areas for tracking control are provided in each of which pilot signal for tracking control are provided in each of which a pilot signal for tracking control is recorded. Further, FIG. 5 shows a digital VTR recording format which is disclosed in Japanese Patent Application Laid Open No. 3-217179 (1991). As shown, the track is divided into a video data area, an audio data area, a servo pilot area, and a sub code area, the pilot signal being recorded in the servo pilot area only.
According to the above construction of the prior art, it is not possible to control the DSV in such a manner as desired, and a separate area has to be reserved for recording a pilot signal for tracking control. Accordingly, accurate tracking control cannot be realized without increasing the data amount and hence increasing the recording rate, which makes it difficult to achieve high density recording.
In conventional digital magnetic recording/reproducing apparatus typified by the rotaryhead digital audio tape recorder (R-DAT), data to be recorded on tracks for recording/reproducing are divided into blocks of identical block length.
FIG. 54 shows the track format and block format employed into conventional R-DAT. In the figure, part (a) shows the track format, and part (b) shows the block format. Referring to FIG. 54(a), the "MAIN DATA" area holds PCM audio data and an error-correcting code associated with the PCM audio data, a total of 128 blocks, each block signal being recorded in accordance with the block format shown in FIG. 5(b). In the "SUBCODE" areas near both ends of the track, subcode data containing additional function information, etc. are recorded as two blocks, each block written in accordance with the same block format shown in part (b) as the "MAIN DATA" area, the same data of two blocks being written a total of eight times per track (four times near the head of the track and four times near the end of the track). The MSB bit of the block address data shown in part (b) is used to discriminate between the PCM audio data and the subcode data both recorded in the same block format.
As described above, in the R-DAT, both PCM audio data and subcode data are recorded in the same block format and subjected to the same signal processing.
The conventional magnetic recording/reproducing apparatus is constructed to record data in the format as described above; whether the data to be recorded is the main data such as PCM audio data or the sub data such as additional function information, the data is constructed into data blocks each having the same information capacity, and the signal processing is fundamentally the same whether in recording or reproducing processes. The advantage of this system is that since the same signal processing circuit can be used for both the main data and sub data, the circuit configuration can be made relatively small in size and is easy to design.
In digital magnetic recording/reproducing apparatus for video such as digital VTRs, the recording track needs to be divided into separate areas, as in DAT recording, for recording video data amounting to several tens of megabits per second (Mbps), audio data needing a bandwidth several tens of times smaller (less than 1 Mbps) than the video data, and sub data having an even smaller information rate (about 100 kbps). If each area is to be recorded using a block structure having the same information capacity, the block structure needs to be determined in accordance with the video data having the largest information rate, in order to ensure sufficient coding efficiency. As a result, in the case of the sub data having a small information rate; either several data units must be combined in one block or redundant data must be added to fill the block. In cases where the information rate is small, such as audio data and subcode data, using a product code as an error-correcting code is not advantageous from the viewpoint of coding efficiency. For such data, it is common to construct the system so that error correction is performed using only an inner block error-correcting code (e.g., subcode in DAT recording). In this case, however, if a burst error occurs within a block in decoding, all information would be rendered unreproducible. Such burst data loss may be prevented by appending redundant data to the sub data and dividing it into a plurality of blocks, but this would in turn greatly reduce the information efficiency since the information rate of subcode data is by far smaller than that of video data.